1. Field of the Invention
The present invention relates to a backlight unit incorporating LEDs as a light source. More particularly, the LED backlight unit of the invention is designed to radiate heat more efficiently from the light source to the outside, reduce constitutional parts, and utilize an inexpensive product for a board where the light source is mounted.
2. Description of the Related Art
A Cathode Ray Tube (CRT) is one of common display devices used in a monitor of a TV, a measuring instrument, an information terminal and the like. However, the CRT cannot actively respond to compactness and light weight demands of electronics products because of its own weight and size.
The CRT has certain restrictions in weight and size and thus does not satisfy the compactness and light weight trend of various electronic products. Accordingly, some optical devices are expected to replace the CRT, which include a Liquid Crystal Display (LCD) based on electro-optic effects, a Plasma Display Panel (PDP) based on gas discharge, an Electro Luminescence Display (ELD) based on electroluminescence and so on. Among these optical devices, the LCD is being most actively researched.
The LCD has been actively developed to replace the CRT because of its merits such as small size, light weight and low power consumption. Recently, development has been made to such a level that the LCD can sufficiently act as a flat-panel display, and thus is being used for a monitor of a lap-top computer, a desk-top computer and a large-area information display. Accordingly, demands for the LCD are continuously in the rise.
Most of such LCDs display images by adjusting the amount of light received from outside and thus need an external light source such as a backlight unit to irradiate light onto an LCD panel.
The backlight unit acting as a light source of the LCD utilizes a cylindrical lamp or Light Emitting Diodes (LEDs). The LED is typically a semiconductor device which produces minority carriers (e.g., electrons or holes) by using a pn junction structure of a semiconductor and then re-combines the carriers to generate light.
In particular, owing to the compactness and slimness trend of information communication devices, various components such as a resistor, capacitor and noise filter are getting more miniaturized. The LEDs adopted as a light source in the backlight unit are also mounted onto a Printed Circuit Board (PCB) via surface mounting or by using lead frames.
In the meantime, the LED backlight unit is classified into a side emitting type and a top emitting type according to light emitting mechanisms.
FIG. 1 is a sectional view of a side-emitting backlight unit with conventional lead-frame type LEDs installed therein. Referring to FIG. 1, the backlight unit 10 includes a lead-frame type LEDs 11 each functioning as a light source, a Metal Core Printed Circuit Board (MCPCB) 12 including a metal member of excellent heat conductivity, a light guide plate 13 for refracting light beams incident from the LEDs 11 to produce a surface light source, a diffuser sheet 14 for diffusing the light beams received from the light guide plate 13 and an LCD panel 15.
Each of the LEDs 11 has lead frames 11a, which are electrically connected to a circuit pattern on the MCPCB 12 via soldering, and a heat conducting member 11b mounted on the MCPCB 12 to serve as heat radiating means. The MCPCB 12 is mounted on a metal chassis 16 with a heat conducting pad 16a interposed therebetween.
A reflector 17 is arranged between the light guide plate 13 and the MCPCB 12 to be parallel with a wall (not shown) extending from the metal chassis 16 in order to reflect light beams generated from the LEDs 11. The reflector 17 has holes 17a perforated for receiving the LEDs 11 without any interference.
FIG. 2 is a sectional view of a top-emitting backlight unit with conventional surface mounting LEDs installed therein. Referring to FIG. 2, the conventional backlight unit 20 includes surface-mounted LEDs 21 each functioning as a light source, an MCPCB 22 including a metal member of excellent heat conductivity, a diffuser sheet 24 for diffusing the light beams received from the LEDs 21 and an LCD panel 25.
Each of the LEDs 21 has leads 21a or electrode terminals, which are electrically connected to a circuit pattern on the MCPCB 22 via soldering, and a heat conducting member 21b mounted on the MCPCB 22 to function as heat radiating means. The MCPCB 22 is mounted on a metal chassis 26 with a heat conducting pad 26a interposed therebetween.
The backlight unit 20 also includes a reflector 2 coated on the top surface of the MCPCB 22. The reflector 2 is made of a reflecting material to reflect light beams incident from the LEDs 21.
In FIGS. 1 and 2, the reference signs 14a and 24a designate a Dual Brightness Enhancement Film (DBEF), and 14b and 24b designate a Brightness Enhancement Film (BEF).
However, in the conventional LED backlight units 10 and 20 as above, since heat generated from the LED 11, 21 is radiated through a path consisting of the heat conducting member 11b, 21b, the MCPCB 12, 22, the heat conducting pad 16a, 26a and the metal chassis 16, 26, the path has a number of interferences between different materials. Then, the efficiency of outwardly radiating high-temperature heat becomes poor in comparison to a case where the heat conducting member 11b directly contacts the metal chassis 16. This as a result accelerates deterioration of the LED package thereby shortening its lifetime and degrading heat radiating characteristics of the backlight units 10 and 20.
Furthermore, a large number of parts are used to make the backlight unit 10, 20. It is also difficult to fixedly locate the reflector 17 having the hole 17a on the wall of the metal chassis 16 or to coat a reflective material on the MCPCB 22 excluding those areas where the LED 21 and other devices are mounted and where the circuit pattern is printed to form a reflector 27. Thus, these problems disadvantageously increase the manufacturing cost of the backlight unit.
Moreover, the MCPCB 12, 22 with the LED 11, 21 mounted thereon is an expensive board having a metal member of excellent heat conductivity, and thus acts as another factor of increasing the manufacturing cost of the backlight unit.